1. Field of the Invention
The present invention relates to a method and apparatus for feedback control of the air-fuel ratio in an internal combustion engine having a plurality of cylinders.
2. Description of the Related Art
Generally, in a feedback control of the air-fuel ratio, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and the detected engine speed, and the base fuel amount TAUP is corrected by an air-fuel ratio feedback amount V(F) which is calculated in accordance with the output signal of an air-fuel ratio sensor (for example, an O.sub.2 sensor or a lean mixture sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio or a predetermined lean air-fuel ratio. According to this feedback control, the center of the controlled air-fuel ratio can be within a very small range of air-fuel ratios around the stoichiometric ratio or the predetermined lean air-fuel ratio.
On the other hand, in an engine having a plurality of cylinders where fuel injection is carried out by electronic fuel injectors each provided for the cylinders, even when each of the fuel injectors are activated by the same injection signal, the fuel amount characteristics of each fuel injector are fluctuated as a result of the individual differences in the parts of the fuel injectors, the long-term changes thereof, and the like. Also, the larger the fuel pressure of a fuel injector, the larger the fuel amount injected by the fuel injector. Therefore, if there are differences in the fuel pressure between the fuel injectors, the fuel amount characteristics of each fuel injector are also fluctuated. Further, the intake air amount characteristics of each cylinder are fluctuated as a result of the individual differences in the structure of an intake air pipe, the opening and closing timing of an intake valve and an exhaust valve, the residual exhaust gas amount within the combustion chambers, the intake air density dependent upon the temperature distribution of the engine, and the like. In addition, the intake air amount characteristics of each cylinder are fluctuated as a result of the fluctuation within an intake air pipe due to the operation of the exhaust gas recirculation system, the ventilation system of a crank case, the evaporation system, the idling engine speed control system, or the like. Thus, in an engine having a plurality of cylinders, it is actually impossible to avoid the fluctuation of the air-fuel ratio between the cylinders due to the fluctuation of the fuel amount characteristics and the intake air amount characteristics thereof.
However, in the above-mentioned prior art air-fuel ratio feedback control system, since air-fuel ratio feedback control is performed simultaneously upon the entire bank of the cylinders, so that the mean air-fuel ratio of the cylinders is brought close to a stoichiometric air-fuel ratio or a predetermined lean air-fuel ratio, the emission characteristics, such as the NO.sub.x component characteristics, the idling stability, and the like are deteriorated, due to the fluctuation of the air-fuel ratio between the cylinders.